As the number of color printers in common use has increased over the past decade, end users of such devices have become aware of the need to recalibrate these devices. Because of the nature of color reproduction, color printers are more subject to drift than are their black and white counterparts. One cause of such drift is deviation of the underlying components due to age or changes in the environment. For example, a typical color printing device is likely to vary due to changes in the underlying marking process in response to external conditions like temperature and humidity, and internal changes due to aging of the components. These variations from a known, standard state, can result in undesirable variation in the appearance of the output from such reprographic devices. Compounding this problem of drift is the sensitivity of human perception, which, for certain colors, emphasizes even minor shifts or differences in color.
While it is possible to include elaborate process controls within the color reprographic device to control its properties, they are often too expensive or cumbersome to be useful in general office reprographic systems. Accordingly, there have been proposed several methods that would allow an end user to recalibrate the printer, that is, restore it to some standard condition so that its reprographic properties may guarantee reliable and consistent color reproduction.
One such calibration method requires users undertake periodic manual calibration processes. In a typical such manual calibration, the user prints out a test page and then examines the page, perhaps visually comparing it to a known sample, to determine what adjustments need to be make. U.S. Pat. No. 5,604,567 to Dundas et al. describes one such calibration process. While such manual recalibration processes help alleviate the process to some extent, they are not as dependable as full feedback process controls, and have the drawback that they rely on the subjective judgement of the user to evaluate the test target. In addition, they are prone to errors if the user enters the recalibration data incorrectly. Systems which rely on use external densitometers, spectrophotometers, or other measuring devices to determine the recalibration are more reliable and can alleviate the problems associated with the manual methods, but require the use of expensive and complex measuring devices that are not normally used in office environments.
As scanners became more in common office environment, and in fact are included in much modern color reprographic devices, several solutions to the problem of calibration using a scanner as a substitute for a densitometer have been proposed. These include, among others, U.S. Pat. No. 5,107,332 to Chan, U.S. Pat. No. 5,452,112 to Wan et al., U.S. Pat. No. 5,537,576 to Sherman et al., U.S. Pat. No. 5,760,913 to Falk et al., U.S. Pat. No. 5,809,213 to Bhattacharya, U.S. Pat. No. 6,141,120 to Falk et al., U.S. Pat. No. 6,185,004B1 to Lin et al., and U.S. Pat. No. 6,215,562B1 to Michel et al. Each of these references describes some way to combine various targets and standards to use with the scanner to develop a way to compensate for drift in the various components of the color reproduction device. However, as a scanner may exhibit some drift due to variations in lamp intensity or aging of the lamp causing a color shift, or the imaging sensor in the scanner may age, many of these methods still require multiple steps to separately calibrate the scanner, or the use of extra auxiliary targets to supplement any calibration targets. Additionally, these methods often can be thus cumbersome to use, time consuming, and may also be dependent on the state of a separate target that must be maintained in good condition to ensure the accuracy of the calibrations produced.
In accordance with the teachings herein there is disclosed a system and method to enable periodically recalibrate a color reprographic device to maintain it in a known state that can be readily performed by a user. In one embodiment, a conventional scanner that is included in the color reprographic device is employed as a colorimeter. The reprographic device includes a calibration conversion table and a stored copy of a color target to be printed by the output portion of the color reprographic device. By scanning the printed target and passing the scanned image through the calibration conversion table, a set of color measurements are obtained which can be used to derives a set of curves that modifies output characteristics of the device to compensate for internal drifts and restore it to a standard, known state. Focusing on maintaining the overall gray balance and linearity of the printer for neutral and near neutral colors has the effect of minimizing the customer perceived variation from its standard state.
In accordance one or more aspects of the teachings herein, there is provided: a color reproduction system with drift correction, comprising: a scanner for scanning a document to generate scanned image data representative of the document, the scanned image data providing a color representation of the document; an output device for generating an output document in response to print ready data; and an image processing system receiving the said scanned image data and generating the print ready data, the image processing system device further comprising a calibration target comprising a set of digital signals representing a plurality of color test patches, a calibration conversion processor for converting the scanned image data into a set of device independent color signals, a calibration processor for computing a set of color shift correction signals by comparing the device independent color signals representative of a printed version of said calibration target with the stored representation thereof, and, an adjustment processor operating to adjust a characteristic of the print ready data in response to said color shift correction signals.
In accordance one or more aspects of the teachings herein, there also provided a method of maintaining the reproduction properties of a color reprographic device. The method comprises causing the device to print a copy of a stored test pattern containing a plurality of colored patches; scanning the printed target with a scanner to obtain a first set of color signals; processing said first set of color signals to obtain average values for the color of each patch in the test target; processing said average values using a color conversion processor to obtain device independent color values for each patch in the test target; comparing the device independent color values to a stored set of standard values; and from the comparison between the measured and the standard values deriving a correction that can be applied to the output means of the color reprographic device to restore it to a standard condition.